1. Field of the Invention
The present invention relates to a transferring device in an image forming apparatus, which transfers a developed image obtained by developing an electrostatic latent image formed on an image carrier to an image receiving medium such as a paper.
2. Description of the Related Art
In image forming apparatuses such as an electrophotographic apparatus or electrostatic printers, devices using an electrostatic transferring method, such as corona transferring or roller transferring and devices using mechanical transferring such as an adhesion transferring, are known as transferring devices. Within these, devices employing a corona transferring method are in wide use.
In recent years, the emission of ozone by corona discharge became a harmful problem. Thus, roller transferring methods, in which there is little generation of ozone have come into use. Also, as the applied voltages used for contact transferring methods, such as the roller transferring method, are lower than those used in corona transferring methods they have advantages in terms of safety. However, although roller transferring methods have this advantage, there are several reasons why they have not become widely used. In roller transferring methods, it is required to press an image receiving medium (paper, etc.) against an image carrier such as a photosensitive drum with a suitable pressure. If this pressure is too light, there will be transfer misses, while if this pressure is excessive, the developing agent (toner) may form blots on the paper and cause transfer failure. Consequently it is necessary to have a high degree of mechanical precision (straightness of about .+-.50 .mu.m) and suitable flexibility (JIS hardness about 10-40 degrees). However, it has been difficult to meet both of these requirements with the conductive rubber used in the prior art. For example, if transfer onto thick paper is effected, excessive pressure is produced and transfer faults occur. Also, differences among many apparatuses arise, depending on the transfer roller or the precision of mounting.
In order to transfer developing agent electrostatically, the material used must prevent destruction of the image carrier by electrical discharge in all environments and maintain a resistance value capable of transferring well and this restricts the possible range of mechanical properties. Other factors which make the selection of material more stringent include the desire to prolong the life of the transferring roller (to, for example, a life of some tens of thousands of transfers) which involves the necessity of cleaning the roller surface and maintaining the smoothness of the roller surface. If the roller is not cleaned, the reverse surfaces of the paper become soiled. Generally, rubber material has a rough surface and its frictional resistance is large. Therefore, although practices, such as dispensing with cleaning and making early replacements of rollers and providing highly smooth material on the roller surface and cleaning it, have been tried. However, if a surface layer is placed on the rubber roller, the resilience of the rubber is adversely affected and there is failure to meet required characteristics. Pricewise the product becomes more costly than a corona transferring device.
Given this background, there is a demand for an image forming apparatus which resolves the above problems and satisfactorily provides all required characteristics.